Published: June 14, 2011 r2011 American Chemical Society 9682 dx.doi.org/10.1021/jp201125k | J. Phys. Chem. A 2011, 115, 9682–9688 ARTICLE pubs.acs.org/JPCA Fast Nuclear Spin Conversion in Water Clusters and Ices: A Matrix Isolation Study Russell Sliter, Melissa Gish, and Andrey F. Vilesov* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States 1. INTRODUCTION It is well-known that H 2 exists in two forms, ortho and para, in which the total nuclear spin angular momentum is I = 1 and 0, respectively. Pure para-hydrogen, p-H 2 , is usually obtained via cryogenic conversion of liquid hydrogen on para-magnetic salts. 1,2 Previous studies demonstrated that gaseous p-H 2 is remarkably stable at room temperature and can be stored for days with negligible back conversion. 3 A number of other molecules such as H 2 O, NH 3 , CH 2 O, and CH 3 F having equivalent hydro- gen atoms are also known to have ortho and para nuclear spin isomers. 4 The possibility of separating nuclear spin isomers in water molecules has attracted considerable attention 58 because of the fundamental atmospheric and biological significance of water. Measurements of the temperature of comets from the abundance ratio of the ortho and para isomers, OPR, have been proposed to determine the temperature of the primordial solar system. 9 Conversely, some recent works have questioned the enhance- ment of para-H 2 O because of its negligible vapor pressure at 30 K and the extent to which the temperature of the water vapor corresponds to the temperature of the comet ice. 10 At equilib- rium and temperatures larger than 50 K, the OPR of H 2 O is very close to the statistical weight of 3:1. Therefore, practical spin conversion requires interaction of the water molecules with a low temperature bath of e10 K. However, the rotation of water molecules is quenched in ices such that the energy difference between the nuclear spin isomers in ice is presumed to be negligibly small. Thus, single water molecules have been isolated in cryogenic rare gas matrices, 1119 where they continue rotating and demonstrate slow nuclear spin conversion at T e 4 K. Nevertheless, controversy remains if para-H 2 O molecules are stable in ice, liquid, or gas. Previous works reported on the short lifetime of p-H 2 O molecules in Ar matrices at higher tem- peratures. 15 Very recently, however, enrichment of ortho- and para-water via gas-phase chromatography has been reported with surprisingly long conversion times in liquid of ∼30 min. 20 In this work, we studied the feasibility of the formation of p-H 2 O ices. We began with the nuclear spin conversion of very diluted samples of water in Ar matrices that have been spin converted at T = 4 K in an enclosed cryogenic optical cell. The ice particles are formed from p-H 2 O molecules upon fast increase in the temperature and removal of the Ar constituency from the cell. Finally, infrared spectra of the gas in the cell have been obtained at T = 260280 K. The acquired spectra are identical to those of unconverted water, showing that the lifetime of p-H 2 O in ice is less than ∼30 min. In addition, our results indicate that spin conversion already takes place in water dimers. 2. EXPERIMENTAL TECHNIQUE The general technique of matrix deposition has been pre- viously described. 18 Our cryogenic system is based on a Janis SHI-4 optical cryostat equipped with a Sumitomo RDK-408D closed-cycle refrigerator. The schematic of the copper cell is shown in Figure 1. The cell is directly attached to the second stage of the refrigerator. The gas is introduced through a stainless steel adaptor using copper tubing. The copper tubing is resis- tively heated to prevent freezing of the water mixture. The tem- perature of the cell was measured by silicone diodes and con- trolled by resistive heating. Ar (Gilmore, 99.9999% purity)) was used without purifica- tion. Water was filtered (Milli-Q) and degassed carefully. H 2 O/ Ar mixtures e1:100 were prepared by standard manometric procedures. Because of adsorption of water onto the stainless steel walls of the gas handling system, the uncertainty in water content is estimated at 50%. The solid sample was prepared by depositing the gas mixture onto the CaF 2 window at a nominal Special Issue: A: David W. Pratt Festschrift Received: February 2, 2011 Revised: May 20, 2011 ABSTRACT: Single water molecules have been isolated in solid Ar matrices at 4 K and studied by rovibrational spectroscopy using FTIR in the regions of the ν 1 , ν 2 , and ν 3 modes. Upon nuclear spin conversion at 4 K, essentially pure para- H 2 O was prepared, followed by subsequent fast annealing generating ice particles. FTIR studies of the vapor above the condensed water upon annealing to T g 250 K indicate fast reconversion of nuclear spin to equilibrium conditions. Our results indicate that nuclear spin conversion is fast in water dimers and larger clusters, which preclude preparation of concentrated samples of para-H 2 O, such as in ice or vapor.